1. Field of the Invention
The present invention relates to printer systems, and more particularly to printer systems of the dot matrix impact type in which indications of the position of a moving shuttle assembly containing printing elements are used to provide signals for periodically energizing selected ones of the print elements to effect printing by impact.
2. History of the Prior Art
It is known to provide a printer system in which a plurality of hammers having dot printing impact tips mounted thereon and which are mounted along the length of a shuttle assembly are selectively actuated as the shuttle assembly undergoes reciprocating motion relative to a print paper or other printable medium so as to impact the paper and print dots thereon. Such a printing system is shown in U.S. Pat. No. 3,941,051 of Barrus et al., which patent is commonly assigned with the present application. In the Barrus et al. printer system the various hammers are subject to actuation at each of a succession of dot printing positions as the shuttle assembly sweeps across the print paper. At each of the dot printing positions each of the various hammers is either actuated or not actuated depending upon the value of a binary data signal associated with that hammer. The dot printing positions are determined by carefully monitoring the location of the shuttle assembly as it reciprocates back and forth across the print paper along a linear path of motion. The shuttle assembly is driven by a cantilevered shuttle drive.
A similar arrangement of a printer system is shown in a co-pending application of Barrus et al., Ser. No. 96,025, COUNTERBALANCED BIDIRECTIONAL SHUTTLE DRIVE HAVING LINEAR MOTOR, which application was filed on Nov. 20, 1979 and is commonly assigned with the present application. In the printer system of application Ser. No. 96,025 a hammer carrying shuttle assembly is reciprocated by an arrangement including a pair of opposite pulleys having an elongated frame which supports the shuttle assembly in contact with the pulleys on one side thereof and an elongated counterbalancing bar in contact with the pulleys on the opposite side thereof from the frame. The shuttle assembly mounting frame and the counterbalancing bar are held in contact with the pulleys by a band which encircles the pulleys and is coupled to the frame and bar and by the attractive forces of a magnet assembly. The magnet assembly combines with the pulleys, frame and bar to form a linear motor in which energization of coils mounted on the counterbalancing bar interacts with magnetic flux flowing through a path including the counterbalancing bar so as to drive the linear motor including the frame and the shuttle assembly in a desired fashion. The shuttle assembly is bidirectionally driven at a desired nominal speed along a linear path of motion between opposite limit positions at which the linear motor engages and rebounds from elastomeric stop members such as springs.
As the shuttle assembly in the printer system of co-pending application Ser. No. 96,025 undergoes movement at the desired nominal speed along the linear path of motion, a encoder is used to provide constantly updated indications of the position of the shuttle assembly along the linear path of motion. The encoder includes a strip extending along a portion of the linear motor and having a plurality of identifiable indicia or "fence posts" spaced apart along the length thereof. A detector mounted in a fixed position relative to the linear motor includes a light emitting diode for illuminating the fence posts and a phototransistor for sensing each passage of a fence post and generating a corresponding fence post pulse. The fence post pulses which define the various hammer firing positions as the shuttle assembly is swept across the print paper constitute the hammer firing pulses and are applied to hammer drivers to actuate selected ones of the hammers in accordance with the data signals associated therewith.
The resulting print or dot density is thus determined by the distance between adjacent fence posts in the encoder. The fence post spacing is chosen in accordance with the desired dot density. Once selected, the dot density remains constant unless the fence post spacing is changed. In order to always maintain the maximum possible print speed, the frequency of hammer firing pulses remains constant, determined by the intrinsic ability of the hammers to recycle. Thus, higher densities require slower shuttle speeds and lower densities allow higher shuttle speeds.
Since it is impractical or generally undesirable to change either the density of the fence posts or the nominal shuttle speed in such printer systems because of the nature of the reciprocating drive mechanism and for other reasons, the dot density remains constant and cannot easily be changed. Changing the dot density usually requires a change in the encoder so that a different fence post spacing is provided. Changing encoders may be difficult if not impossible for the average user of the printer system. In any event, it prevents the changing of print density on a rapid basis.
Accordingly, it would be desirable to provide a printer system in which the print density can be relatively quickly and easily changed to various different values and at each density to fire the hammers at the maximum rate at which they are capable of being reliably and repeatedly fired. Such an arrangement would enable a given printer system to be shared by different users desiring different print densities, and corresponding effective printing speeds, or by the same user desiring different print densities for different printed items. Changes in the print density should desirably be accomplished electronically not only for reasons of speed but to avoid the need to physically enter and make changes within the printer system.